In recent years, liquid crystal display devices that display an image on a display panel using a backlight are being widely used in a variety of industrial fields. For example, FPDs (Flat Panel Display) that display a video signal supplied from the exterior of the liquid crystal display device on a display panel using liquid crystals are being frequently used. Such liquid crystal display devices include a display panel using a liquid crystal element and an optical member of a backlight (light source, BL: Back Light) such as an extra-high pressure mercury lamp, a W/RGB LED (Light Emitting Diode), or a cold-cathode tube (CCFL: Cold Cathode Fluorescent Lamp), that irradiates light to the display panel. Furthermore, the display device includes, as a circuit part, a BL driving part that controls the brightness of the light emitted by the backlight, and a circuit that drives the display panel.
In a case where the display device mentioned above is used as a display device in graphic design or for medical applications, it is required that the display characteristics (stability of the color display) specified beforehand be maintained over a long period, and for it to represent a display quality that can be utilized in a state where it is constantly stable.
There exists a display device that, when this stable state is to be maintained, is provided with an optical sensor, such as a brightness sensor or a color sensor, on the display surface of the display panel, that stabilizes the display quality and measures the brightness or the color, and performs adjustments corresponding to the display characteristics according to the measured values (for example, refer to Patent Document 1). In a case where the optical sensor is provided on the display surface of the display panel, it becomes possible to measure not only the brightness value, but also to measure the displayed color and the gamma (γ) characteristics of the display panel in order to maintain the display quality.
On the other hand, there also exists a display device that is provided with an optical sensor or the like, not on the display surface but on the back surface of the display panel, that stabilizes the display quality by means of the measured values. For example, in a case where the display device is a liquid crystal display device, if an optical sensor is provided on the back surface of the liquid crystal panel (display panel), the brightness of the light irradiated is stabilized by measuring the brightness of the light irradiated by the backlight to the liquid crystal panel.
Therefore, a configuration that provides the optical sensor on the display surface of the display panel has an advantage compared to a configuration that provides the optical sensor on the back surface of the display panel, from the point that it is able to better perform the stabilization of the display quality.
Next, FIG. 6 is a diagram showing a configuration example of a liquid crystal display device 200 of the configuration mentioned above that provides the optical sensor on the back surface of the display panel.
An optical sensor 205 measures the brightness of the light irradiated by the backlight 3 to the back surface of a display panel 2, and outputs the measurement result as a measured optical value to a signal processing unit 204.
The signal processing unit 204, by means of the measured optical value supplied from the optical sensor 205, converts the gradient of an input video signal that is input from an external device into a post-processing video signal that drives the display element of the display panel 2, and outputs it to the display panel 2. Furthermore, the signal processing unit 204 generates a drive signal such that the brightness value of the light emitted by the backlight 3 becomes a set brightness value that is preset in an internal storage part, that is to say, the brightness value of the light irradiated becomes constant even if the temperature changes, and performs control of the backlight 3 by means of the drive signal.
Next, FIG. 7 is a diagram showing a configuration example of a liquid crystal display device 300 of the configuration mentioned above that provides the optical sensor on the display surface of the display panel.
In a case where an optical sensor 5, for example, represents a brightness sensor, it measures the brightness of the light transmitted through, and emitted from, a measurement area 21 of a display panel 2, and emitted by a backlight 3 to the display panel 2, and outputs the measurement result as a measured optical value to a signal processing unit 304.
The signal processing unit 304, by means of the measured optical value supplied from the optical sensor 5, converts the gradient of an input video signal that is input from an external device into a post-processing video signal that drives the display element of the display panel 2, and outputs it to the display panel 2. Furthermore, the signal processing unit 304 generates a drive signal such that the brightness value of the light emitted by the backlight 3 transmitted through the measurement area 21 becomes a set brightness value that is preset in an internal storage part, that is to say, the brightness value of the light irradiated becomes constant even if the temperature changes, and performs control of the backlight 3 by means of the drive signal.
At this time, the signal processing unit 304 measures a black brightness level and a white brightness level, and calculates a drive signal with respect to the backlight 3 and a post-processing video signal such that the dynamic range becomes a preset numerical value. That is to say, by controlling the degree of opening of the display element in the measurement area 21, the brightness value transmitted from the measurement area 21 corresponding to the degree of opening can be measured. Furthermore, as mentioned above, it is possible to measure not only the white brightness level, but also the black brightness level or the brightness values of a plurality of halftones (gradients between the white brightness level and the black brightness level) for determining the gamma characteristics.
In the liquid crystal display device of FIG. 7 mentioned above, in a case other than a measurement that makes constant the brightness value of the light emitted from the display panel 2 and stabilizes the display quality, for example when a color measurement, a gamma characteristics measurement, or a measurement of the black brightness level or of a halftone is performed, it is necessary for the signal processing unit 204 to disable the brightness stabilization function. The brightness stabilization function generally measures the white brightness level by making white be displayed at the measurement area 21, and controlling the brightness value of the light irradiated by the backlight 3 such that the brightness value becomes constant. That is to say, the brightness stabilization function controls the drive signal that drives the backlight 3 such that the brightness value of the white brightness level becomes a preset set value.
Consequently, in a case where a measurement other than that of the white brightness level is performed, if the brightness value is not measured in a state where the brightness stabilization function is disabled, for example if a measurement of the brightness value of the black brightness level is performed, the signal processing unit 304 reads the brightness value of the black brightness level as the brightness value of the white brightness level. Then the signal processing unit 304 controls the backlight 3 and the display panel 2 according to the brightness value of the black brightness level such that it becomes a preset set value, that is to say, the brightness value of the white brightness level. Therefore, the brightness value of the light transmitted from the display panel 2 is not correctly controlled.
Consequently, in a case where a measurement of a brightness value other than that of the white brightness level is performed, the measurement process is performed with the brightness stabilization function mentioned above disabled.
However, in a case where the brightness stabilization function is disabled, color measurements, gamma characteristic measurements, and measurements of the black brightness level or of the brightness values of halftones are performed without performing control of the brightness value of the light irradiated by the backlight 3. Then, after the signal processing unit 304 completes the processing for stabilizing the display quality, that is to say, after the signal processing unit 304 has completed the color measurement, the gamma characteristic measurement, or the measurement of the black brightness level or of the brightness value of a halftone, it makes active the brightness stabilization function, and changes from a measurement mode for performing optical measurements of the display quality to a normal mode for performing a normal image display.
At this time, if the brightness stabilization function is disabled, there is a concern of the brightness value of the light transmitted through the display panel 2 changing. For example, in a case where a CCFL is used for the backlight 3, since the luminous efficiency of the CCFL changes with the temperature, if the illumination time is long the temperature rises and hence the brightness value of the light irradiated to the display panel 2 changes.
Next, FIG. 8 is a diagram explaining the brightness value of the light irradiated by the backlight 3. In FIG. 8, for example in a case where the brightness value of the light irradiated by the backlight 3 is stable even after time has elapsed, the brightness value L0 measured at the time T0 and the brightness value L1′ measured at the time T1 become equal. However, in a case where the brightness value of the light irradiated by the backlight 3 changes with the elapsing of time, the brightness value L0 measured at the time T0 and the brightness value L1 measured at the time T1 are not equal.
Therefore, when the brightness stabilization function is disabled and the gamma characteristics or brightness values, such as of a halftone or the black brightness level, are being measured, a displacement in the brightness value occurs depending on the time at which the measurement is performed as a result of changes in the brightness value of the light irradiated by the backlight 3 to the display panel 2. That is to say, at the time of a measurement of the measured optical value for performing stabilization of the display quality, if a video signal to be made the same gradient is provided to the display panel 2 and the measurement is performed at a different time, although the control value of the gradient is the same, different brightness values are measured as the measured optical values each time. Consequently, the measured optical value that is measured for adjusting the set value that controls the display quality in a state where the brightness stabilization function is active becomes measured each time as a value that is displaced with respect to a state where the brightness stabilization function is active. As a result, by using each of the measured optical values measured at different times after the brightness stabilization function is disabled and respectively comparing them, an adjustment of the image quality control set value, which performs image quality control in a state where the brightness stabilization function is active, becomes performed. Consequently, the accuracy of the display quality stabilization process when the brightness stabilization function is active, which is performed using an image quality control set value that is adjusted by measured optical values having respectively different displacement amounts compared to a state where the brightness stabilization function is active, becomes lower.
Therefore, in the configuration of the liquid crystal display device 300 using a configuration in which the optical sensor is provided on the display surface of the display panel, only a low-quality control of the display quality can be performed for users that require an accurate display image quality and a stable display quality.